Can Someone Help Me With My Mob Farm? [Minecraft Bedrock Edition]

It's hard to say for sure without seeing the mob farm and since I play on Java, but I can try to make some guesses. If you covered it using half slabs instead of full blocks, it might be letting light in that's stopping things from spawning. Assuming it works the same way as Java, there might also be things spawning below it that are filling the mob cap for the chunk you built in. In order for it to spawn as many mobs as possible, you would need to fully light up the area outside the spawner, including caves underground. Other than that, sometimes it takes a while for them to start working so you might just need to wait a while.

Edit: also make sure you're not standing directly next to it since mobs can only spawn a certain distance away from a player.